Yao N. Focused Ion Beam Systems: Basics and Applications

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semiconductor industry for mask repairing, device modiﬁcation, failure

analysis, and integrated circuit (IC) debugging, but also a popular tool in

making high-quality and high-precision nanostructures [1–3].

Two basic schemes, projection printing and direct writing, are used in IB

nano-fabrication. In projection printing, a collimated beam of ions passes

through a stencil mask and the image of the mask is projected onto the

substrate using electrostatic lens systems, while in direct writing, a small spot

of the focused ion beam (FIB) is struck directly onto the substrate without a

mask. The technique of ion projection printing, also known as ion projection

lithography (IPL), which originates from the semiconductor industry,

enables parallel production of a large number of devices and has been one of

the major candidates for next-generation lithography (NGL) to replace the

currently used optical lithographic technique. Other types of printing

schemes, including proximity printing and contact printing, are not included

in this chapter because they offer limited ﬂexibility and no rigorous devel-

opment efforts have been reported for future industrial applications. The IB

direct writing is a collection of several techniques, including milling,

implantation, ion induced deposition, and ion assisted etching, which remove

materials from, change properties in, or deposit materials on a substrate by

direct impingement of the ion beam on the target with or without chemical

reactions.

In this chapter, both projection printing and direct writing techniques are

explored for their abilities in fabricating nanostructures. In projection

printing, the current developments in its equipment, ion source, masks, and

resists with focus on their applications for NGL are assessed. The nanos-

tructures resulted from different exposures and trial tests are presented to

illustrate their feasibility and resolution. In direct writing, the ion source

and equipment for FIB are ﬁrst assessed; the principles and techniques of

four FIB-related processes are then introduced and evaluated separately.

The associated nanostructures made by different procedures are also pre-

sented in order to illustrate the versatility and advancement of the FIB-

related techniques. Finally, concluding remarks with recommendations are

provided.

7.2 Ion projection lithography (IPL)

Lithography is the process of transferring patterns from one medium to

another. In this section, recent development related to the technology of IPL

is presented.

Focused ion beam systems188

7.2.1 Ion source

Two types of ion sources, either point or volume-plasma sources, have been

developed to produce nanometer resolution patterns. The volume-plasma

sources are normally used for IPL, where a collimated ion beam is formed

and projected on a substrate or resist through a mask with or without

demagniﬁcation. On the other hand, the point source is used to shape an IB

for direct writing.

In general, the axial energy spread of the ion beam, when coupled with the

chromatic aberration in the ion optical column, can lead to blurring in the

printed pattern on the target. The multicusp volume-plasma source has

replaced the duoplasmatron volume source for ion projection printing

because its ability to provide a lower axial-energy spread of ions minimizes

the chromatic aberration of the projected image. Also, the multicusp source

can be used to produce large volumes of uniform, quiescent, and high-density

plasmas with high gas and electrical efﬁciencies. The multicusp source is

based on electron-impact ionization, in which the energy transferred to a gas

molecule from an energetic electron exceeds the ionization energy by means

of ionizing collisions. Both the electrons and excited ions are accelerated by a

dc ﬁeld or rf power at a frequency of a few MHz (up to 13.56 MHz) and

conﬁned by an imposed magnetic ﬁeld. The multicusp source is generally

used to produce hydrogen and helium ion beams. Other ion beams of Ne, Ar,

and Xe can also be generated.

An international IPL development program, called MEDEA, led by Inﬁ-

neon and Sematech, has used a coaxial multicusp ion source for its newly

built IPL system, also known as the process development tool (PDT) for

testing the virtual source size, energy spread, homogeneity, total extract

current, and life time of the source [4]. With a specially designed extraction

system, the PDT can extract a 5 keV He

beam to yield an axial energy

spread as low as 0.6 eV. Consequently, this ion source can be expected to

produce sharp features and to achieve the 50 nm resolution target. The light

ions (H

,He

) are particularly suitable for projection printing

because they have very little forward scattering and give off very little energy

to the secondary electrons in the polymeric resist.

7.2.2 IPL system

In an IPL system, a collimated beam of ions passes through a mask and is

accelerated by an electrostatic lens to create a demagniﬁed image of the mask

on the wafer. The wafer is stepped chip by chip. Many efforts have been

Fabrication of nanoscale structures using ion beams 189

devoted to the advancement of IPL technologies for the applications below

100 nm. In the MEDEA project, a 4 · reduction ion optics has been devel-

oped for its prototype process development tool (PDT), in which a printing

area of 12.5 mm · 12.5 mm is designed to be printed on the wafer. To obtain a

full image of the wafer, each printing area is stitched one-by-one via syn-

chronization of the beam and wafer moment [5]. The PDT has been used to

investigate the industrial suitability for mass production of next-generation

chips and is one of the major candidates for NGL to replace the conventional

photolithography.

The co-axial multicusp ion source used in PDT has a very low energy

spread at the 1 eV FWHM (full-width-half-maximum) level. Multi-electrode

electrostatic ion optics has been used as the diverging electrostatic lens while

an online diagnostic system and ﬁeld composable lens are used to compensate

for mechanical manufacturing inaccuracies. Figure 7.1 shows a schematic of

the PDT ion-optical system. The multicusp ion source is equipped with a

co-axial Wien (EXB) mass ﬁlter. A multi-electrode electrostatic condenser

lens illuminates the 150 mm stencil mask with a nearly telecentric ion beam of

115 mm in diameter. After exciting the stencil mask, the ion beamlets are

further accelerated to energies in the 70–150 keV range and then demagniﬁed

into a parallel beam whose image is focused at the wafer. A nine-element

multi-electrode reduction optics is used to accelerate and demagnify the

beamlets, while a pattern lock system monitors the position of 12 reference

beamlets as they travel through the ion-optical system. Diagnostic elements

are provided to measure the energy spread, beam uniformity, and distortion.

Vacuum

Ion

source

Injection

unit (ExB)

Stencil

mask

Condenser

ion optics

Turbo

pump

4× reduction ion optics

1.9 m

150 mm wafer exposure station

and in-situ distortion measurement

Si wafer

vacuum

lock

Metrology stage

Pattern lock system

Diagnostic systems:

• energy spread

• beam homogeneity

Cryo

Figure 7.1 Schematic of PDT system. (Reprinted from [13] with permission

from American Institute of Physics.)

Focused ion beam systems190

The uniformity of the current density can be controlled within 3%. By system

optimization, it is expected that the uniformity can be achieved within 1%.

The Lawrence Berkeley National Laboratory has developed a maskless ion

projection printing system called maskless microion-beam reduction litho-

graphy (MMRL). As reported by Ngo et al. [6], MMRL consists of a co-axial

multicusp ion source, a multibeamlet pattern generator, and an all-electrostatic

reduction ion-optical column. The pattern generator is used to create litho-

graphic patterns to eliminate the need for masks. During processing, each

individual ion beamlet can be switched on or off to form the lithographic

pattern by biasing the extraction electrode with respect to the plasma electrode.

Removing the use of stencil masks and the ﬁrst stage, normally required by the

other IPL systems (such as PDT), can eliminate the cost and efforts for mask

development and fabrication as well as having a great potential to reduce the

equipment and operation cost. Jiang et al. [7] and Ngo et al. [6] have used nine

50 mm switchable apertures to generate the beamlets with 10 · reduction ion

optics to demonstrate the proof-of-concept of MMRL and concluded that the

maskless system is a vital candidate for NGL.

7.2.3 Mask and resist

Most of the IPL systems use two types of masks for pattern transformation as

indicated by many investigators [5,8,9]. The mask needs to be made from an

ion-transparent substrate with a pattern made up of an absorber surface. The

ﬁrst one can be made of a single layer of silicon that uses a varying thickness of

the silicon acting as the absorber and the transparent membrane. Using the

same single silicon crystal minimizes the problem created due to the thermal

distortion and proton scattering in conventional masks. This is relatively easy

to manufacture using the conventional IC processes. The second type, known

as the stencil or open mask technology, is made up of open masks, in which the

patterns are etched on the metal foils. This type of mask has excellent contrast,

as the ions are not affected by passing through the open hole before striking on

the resist. However, since there is no sub-layer to hold the etched metal pat-

tern, every section in the etched pattern has to be connected to each other and

no island or detached sections can be included in the pattern. As a result, the

second type can only be used for very limited geometries.

Deep-ultraviolet (DUV) chemical-ampliﬁed resists and polymethyl metha-

crylate (PMMA) are the two most popular types of IPL resists [5,6]. They

have the best combination of resolution, minor sensitivity, and etch resistivity.

Other resists used for IPL include Hoechst’s AZ series, Olin’s HPR 506,

OCG’s HPR and ARCH [9,10]. By applying the Top Surface Imaging (TSI)

Fabrication of nanoscale structures using ion beams 191

principle and using Ga ion beam exposure associated with silylation and

oxygen dry etching, a DNQ (diazonaphthoquinone)/novolak-based resist

pattern can be obtained to be as small as 30 nm, yet maintaining a high aspect

ratio of up to 15 as indicated by Arshak et al. [ 11].

For a PDT demonstration, 150 mm stencil masks have been developed and

fabricated on SOI (silicon-on-insulator) mask-wafer blanks using established

semiconductor fabrication processes. The silicon membrane is 3 mm thick and

coated with a 500 nm thick carbon protection layer to ensure adequate mask

life. The design of these masks contains an array of overlay measurement

structures. Through PDT’s 4 · reduction ion-optics, one of the overlay test

masks having an array of I-marks with a linewidth of 400 nm (Figure 7.2(a ))

has been successfully used to print a 100 nm wide I-marks pattern in a 240 nm

thick Shipley XP9946-D resist as shown in Figure 7.2(b ) [ 5]. A 37.5 keV He

ion beam at an exposure dose of 1.35 m C/cm

is used to project the I-marks

pattern on this Shipley DUV chemical ampliﬁed resist, which exhibits a sen-

sitivity of 0.8 mC/cm

fo r 3 7. 5 ke V He

io ns [ 12 ]. This and other tests have

demonstrated that mask-to-wafer transfer for isolated and arrayed lines/space

can be performed within the required accuracy. Also, as reported by Lo

es ch ne r

et al. [ 13 ], 75 nm feature patterns can be achieved and pattern collapses are the

limiting factor for determining the ultimate resolution of the system.

Moreover, exposures have been performed using the ion projector IPLM-

02 manufactured by Ion Microfabrication Systems (IMS) on the Shipley UV II

HS resist [ 14]. A 3.5 keV He

ion beam is used and accelerated behind the mask

to 75 keV. The resist used is 180 nm thick and has been diluted so that a smaller

resist thickness of 180 nm could be obtained. For a 75 keV H

ion exposure, the

(a) (b)

Sl-OTM-TF2-01-FS3,7-RTP-x-y-0,4-3, 11h,10kV, Wpt 3 µm

POT CO

µm-3/4, 30k, 10kV, W 1 µm

Figure 7.2 Mask-to-wafer transfer by PDT. (a) SEM image of 400 nm

resolution patterns of overlay test mask (modiﬁed from [ 5 ]), (b) 100 nm line-

space pattern printed in 240 nm thick Shipley XP9946-D resist. (Courtesy of

Hans Lo

eschner of IMS Nanofabrication in Vienna.)

Focused ion beam systems192

resist sensitivity is 10

ions/cm

, which corresponds to 0.15 m C/cm

. An open

stencil mask with arrayed patterns fabricated on a SOI wafer is used for a

resolution study. Figure 7.3(a) shows the stencil mask with a linewidth of

650 nm, while, through an 8.7 demagniﬁcation by ion optics, a pattern of

75 nm wide lines and spaces is printed on the DUV resist without pattern

collapse as shown in Figure 7.3 (b). As indicated in Figure 7.3, the mask

shows good edge quality, while no proximity effect is visible at the line ends

in the exposed resist pattern, even though the resist has been removed by a

second exposure of equal dose in the front part of the picture to allow SEM

side view. It is noteworthy that the higher dose used in the test could effec-

tively reduce the edge roughness of the resist.

7.2.4 Resistless IPL

Ions have a unique feature of directly modifying a wide range of materials

without the need of a resist. In IPL, a whole surface area can be treated in

parallel and can be patterned into a substrate in a single-step process that

should be much faster than the series process or direct writing using FIB. IPL

has been used for the resistless patterning of semiconductors (Si, GaAs, poly-

Si), insulating layers (SiO

,Si

), and metals (Al, Ni, Mo, Au) in addition

to patterning organic thin ﬁlms and resists. In many conditions, it is desirable

to avoid the use of a resist. For instance, in making high-temperature

superconductor nanostructures, undesirable chemical reactions could occur

with the resist.

Bruenger et al. [ 15] have used the IPLM-02 ion projector to perform

direct milling on a 35 nm thick Au ﬁlm using an open stencil mask with 8.7

(b)(a)

000005 25KV X6.00K 5.0um

.. . . . . . .. . .

000028 25KV X40.0K 0.75um

.. . . . . . . . . .

Figure 7.3 Ion projection resolution evaluation. (a) SEM image of stencil

mask with 650 nm wide lines and spaces, (b) SEM image of 75 nm line

pattern printed by 8.7 · reduction in 180 nm thick Shipley DUV resist.

(Courtesy of W. H. Bruenger of Fraunhofer Institute in Berlin.)

Fabrication of nanoscale structures using ion beams 193

demagniﬁcation ion optics. To increase the milling rate, the light ions

normally provided by the multicusp ion source used in IPLM-02 are

replaced by the heavier Xe

species without any change in emission sta-

bility or uniformity. Figure 7.4(a) shows an array of line patterns milled

with a dose of 2 · 10

ion/cm

at 75 keV having the smallest linewidth

of 130 nm, while, based on the proﬁle measurement by white light inter-

ferometry shown in Figure 7.4(b), the milling depth is 8 nm. The apparent

roughness of the gold ﬁlm is believed to be due to its grain structure.

IPL milling should be especially attractive for fabricating magnetic nano-

dots, since other techniques, including photolithography, e-beam lithography,

and nanoimprint lithography have to use resist-based processes that are not

preferable to keep the topography of a surface unchanged. This is extremely

important for magnetic disks with a surface roughness of a few nanometers.

Through a stencil mask, Dietzel et al. [ 16 ,17] used the IPLM-02 projector

with a 3 · 10

io n/ cm

dose at 73 keV to mill magnetic Co-Pt multi-

layers. Based on the magnetically altered areas measured by magnetic force

microscopy (MFM), Dietzel et al. [16,17 ] reported that magnetic islands

with an averaged diameter of less than 100 nm are formed. Also, the atomic

force microscopy (AFM) measurement indicates that the surface roughness of

the topography after the IPL process is 1.1 nm in rms, which conﬁrms that

the topography change by IPL is negligible. Thus, IPL is acceptable for

magnetic media applications. Earlier, Bruenger et al. [ 15 ] had also used the

IPLM-02 projector to mill a magnetic Fe-Pt ﬁlm with a 10

io n/ cm

dose

at 75 keV and reported that the averaged magnetic island size was 340 nm. By

experimental evidence and numerical simulation, Dietzel et al. [16]and

Bruenger et al. [15,18] noticed that Xe

and Ar

are more effective by two

orders of magnitude compared to He

and believe that with an appropriate

µm140120100806040200

–4

–8

(a) (b)

Figure 7.4 IPL milling with Xe

ion in polycrystalline Au ﬁlm. (a) SEM

image of milled line pattern with smallest width of 130 nm, (b) depth proﬁle

of milled pattern measured by white light interferometry. (Courtesy of

W. H. Bruenger of Fraunhofer Institute in Berlin.)

Focused ion beam systems194

mask design, IPLM-02 can make magnetic dots as small as 50 nm in diameter,

which would result in a storage density greater than 10 Gbit/cm

7.3 Direct writing

FIB is normally used for the direct writing of nanostructures. IB direct

writing is a process of transferring patterns by using an energetic FIB to

directly hit the target to cause physical and/or chemical changes in the target

materials. Four major direct writing processes will be considered in this

section milling, implantation, ion assisted etching, and ion induced deposi-

tion. While the ﬁrst two are governed by physical alterations, the latter two

are dominated by chemical transformation.

7.3.1 Ion source

FIBs use liquid-metal ion sources (LMIS), which are high brightness ion

sources because they can produce a beam of heaver ions that can be focused

into ﬁne spots of the order of 10 nm with adequate current densities for direct

writing. Almost all metals that have relatively low melting temperatures and

low reactivity can be used as the sources. The range of materials being used in

FIB systems is also expanding to further increase the extent of their appli-

cations. The ion sources that are currently available include Al, As, Au, Be,

Bi, Cs, Cu, Ga, Ge, Er, Fe, In, Li, Ni, Pb, Pd, Pr, Pt, U, and Zn [1]. Among

these, Ga is the most popular ion species used in FIB for direct writing. In

order to lower the melting point and to control the reactivity, alloy sources,

such as PdAs, PdAsB, AuSi, and AuSiBe, are frequently used to deliver the

dopants for semiconductors [19].

A typical LMIS consists of a capillary tube with a needle through it, an

extraction electrode, and a shielding. The capillary acts as a reservoir that feeds

liquid-metal to the tip. The interaction of the strong electrostatic force gen-

erated by the extraction electrode and the surface tension causes the liquid-

metal meniscus to form a sharply peaked cone, also known as the Taylor cone.

The application of the critical Taylor voltage on the liquid-metal cone extracts

positively charged ions. The ions are condensed into focused parallel beams by

the lens and ground electrodes, also called the upper or condenser lens [1].

7.3.2 FIB systems

The basic components of an FIB system consist of an ion source, ion optics, a

substrate stage, and a vacuum chamber with auxiliary equipment. Figure 7.5

Fabrication of nanoscale structures using ion beams 195

schematically shows an FIB system with an LMIS. The ions are focused and

collimated into parallel beams by the upper (condenser) lens. Then, the ion

beam is passed through a mass separator and a drift tube. A mass separator

is only necessary for high-energy systems equipped with liquid alloy ion

sources. It is set up to ﬁlter out unwanted ion species emitted from the alloy

ion sources by allowing only the required ions with a ﬁxed mass–charge ratio

to pass through. Below the mass separator, there is a long collinear drift tube

equipped with a stigmatic focusing lens for stigmatism and collimation,

which eliminates the ions that are not directed vertically. The lower (objec-

tive) lens is located below the drift tube and helps in reducing the spot size

and in improving the focus. Following the objective lens is an electrostatic

beam deﬂector that controls the ﬁnal trajectory or landing location of the

ions on the substrate.

Many accessories, including the nozzle shown in Figure 7.5, are equipped

for FIB assisted etching and induced deposition. Also, in processing insu-

lated substrates, a nozzle type of equipment is needed to provide low energy

nonfocused electrons to compensate the charging on the substrate surface

and to neutralize the substrate. Often, a multi-channel plate (MCP) is

located above the target. The MCP helps in recording the secondary electron

emission and thereby, helps in viewing the substrate. All the components are

usually placed in a low-pressure chamber evacuated to the 10

7

Torr regime.

Source

(LMIS)

Substrate

Nozzle

Drift tube

Mass

separator

Upper

lens

Lower

lens

Beam

deflector and

MCP

Figure 7.5 Schematic of FIB system.

Focused ion beam systems196

This is done so that the mean free paths of the ions are increased and the

strength of the beam is not reduced due to the interference of the particles in

the chamber [1,20].

7.3.3 FIB milling

Essentially, milling is a process combining physical sputtering, material

redeposition, and amorphization. In this section, following the evaluation of

sputtering, the effects of redeposition and amorphization are discussed. The

combined effects in milling are then illustrated by evaluating the milling

characteristics of three basic nanostructures: holes, trenches, and 3D stacked

structures. An understanding of these milled patterns can lead to enhancing

the material removal rate in milling and further improving the dimensional

controllability for milling high-precision, more complex nanostructures.

Sputtering and sputter yield

When an energetic ion bombards the surface of the target solid, a variety of

ion–target interactions can occur. The most important interaction for milling

is sputtering, in which the energy transferred from the ions to the targeted

substrate is high enough leading to a collision cascade involving substrate

atoms on or near the surface to cause material removal. For the majority of

the ion sources used in FIB, the optimized ion energy leading to a surface

collision cascade of most engineering target materials is in the range from 10

to 100 keV. For energy higher than the order of 100 keV, the ions can easily

penetrate into and be trapped inside the target substrate where the implan-

tation occurs. For ion energy higher than the order of 1 MeV, inelastic

interactions can become dominant and many high-energy particles can be

generated. In milling, only the sputtering interaction is desirable.

Normally, the sputter yield is used to measure the efﬁciency of the sput-

tering process and is deﬁned as the number of atoms ejected from the target

surface per incident ion. A software package, called SRIM (Stopping and

Range of Ion in Matter), has been widely used for predicting the sputter yield

for many different ions over a wide range of energies. SRIM is a compre-

hensive program that uses a Monte Carlo treatment of ion–atom collisions to

calculate the stopping range of ions (10 eV to 2 GeV/atomic mass unit) into

matter [21,22]. It provides the distribution of the ions and the kinetic phe-

nomena associated with the ion’s energy loss, including target damage,

phonon production, ionization, ion reﬂection, implantation, and sputtering.

The dependence of the sputter yield on ion energy, incident angle, and the

target material can be predicted by SRIM. Figure 7.6 shows the sputter yield

Fabrication of nanoscale structures using ion beams 197